1. Field of the Invention
The present invention relates to a nonvolatile memory device and a method of driving the same, and more particularly, to a nonvolatile memory and a method of driving the same, which adopt an improved write verify operation.
2. Description of the Prior Art
A nonvolatile memory device using a resistance material includes a phase change random access memory (PRAM), a resistive RAM (RRAM), a magnetic RAM (MRAM), and the like. A dynamic RAM (DRAM) or a flash memory device stores data using charges, whereas the nonvolatile memory device using the resistance material stores data using a phase change of a phase change material such as a chalcogenide alloy (in the case of a PRAM), a resistance change of a variable resistance material (in the case of an RRAM), a resistance change of a magnetic tunnel junction (MTJ) thin film according to a magnetization state of a ferromagnetic material (in the case of an MRAM), and the like.
More specifically, the PRAM stores data using a phase change material such as a chalcogenide alloy of which the phase is changed to a crystalline state or an amorphous state as it is cooled after being heated. Since a phase change material in a crystalline state has a low resistance and a phase change material in an amorphous state has a high resistance, the crystalline state may be defined as a set state or a logic level “0”, and the amorphous state may be defined as a reset state or a logic level “1”.
The PRAM provides a set pulse or a reset pulse to the phase change material, and performs a write operation using joule heat generated according to the set pulse or the reset pulse provided to the phase change material. Specifically, the PRAM makes the phase change material in an amorphous state by heating the phase change material over its melting point using the reset pulse and then quickly cooling the melt phase change material, or makes the phase change material in a crystalline state by heating the phase change material to a temperature, which is higher than the crystalline temperature, but is lower than the melting point of the phase change material, and then cooling the phase change material after maintaining the temperature for a preset time.
In order to prevent unnecessary current consumption, the nonvolatile memory device using the resistance material including the phase change memory device may adopt a write verify operation. The write verify operation is performed in a manner that, before write data is written, data pre-stored in variable resistance memory cells (e.g. phase change memory cells in the case of a phase change memory device) is read (hereinafter referred to as verify data), the verify data and the write data are compared with each other, and the write data is written only in fail variable resistance memory cells in which the verify data and the write data are different from each other. When the write verify operation is performed, it is required that resistance values of variable resistance memory cells are arranged in specified resistance windows.